The present invention relates to a new process for the conversion of organic materials, particularly saccharide materials, comprising an oxidation step carried out under particular conditions, namely combining, at least at a given moment, a means of enzymatic oxidation capable of generating hydrogen peroxide and at least one particular metal, namely ruthenium and/or palladium.
The invention also relates to a process as described above comprising an additional step involving the oxidation or reduction of the material oxidised enzymatically beforehand.
In particular, the claimed process makes it possible to obtain, in a simple, rapid and inexpensive manner, organic materials, particularly those of a saccharide nature, oxidised with high selectivity then optionally reduced or re-oxidised, having numerous industrial applications, including the use as synthesis intermediates, such as glucosone, galactosone, gluconic, 2-keto-gluconic or isoascorbic acids, fructose, sorbitol, mannitol etc.
The term xe2x80x9corganic materialsxe2x80x9d within the meaning of the present invention means both saccharide materials and non-saccharide organic materials.
The latter include alcohols and organic acids of a non-saccharide nature such as, for example, lower alcohols including methanol, fatty alcohols, glycerol, cholesterol, polyvinyl alcohols, hydroxycarboxylic acids including malic acid, the respective derivatives thereof and, generally, all organic products other than saccharides which are potentially capable of being oxidised enzymatically with the concomitant production of hydrogen peroxide.
As mentioned, the process forming the subject matter of the invention may be applied advantageously to saccharide materials, this idea being in no way limiting and including linear, cyclic or branched monosaccharides, disaccharides, trisaccharides, oligosaccharides and mixtures of these products such as hydrolysates of starch, insulin or cellulose.
It may also be applied to saccharide materials which have undergone, before the characteristic step of enzymatic oxidation, at least one step involving chemical, enzymatic and/or physical modification, particularly hydrolysis, oxidation or hydrogenation and/or at least one purification step.
Preferably, the saccharide material is chosen from the group comprising monosaccharides, disaccharides, oxidised or hydrogenated derivatives of monosaccharides and disaccharides, and any mixtures of at least any two of these products, independently of the process by which such products were obtained.
Monosaccharides may consist, in particular, of pentoses or hexoses such as xylose, arabinose, ribose, glucose, galactose, mannose, sorbose or fructose.
Disaccharides may consist, in particular, of maltose, isomaltose, lactose, lactulose, cellobiose or sucrose.
As mentioned, the process according to the invention may also be applied to organic materials composed of purified or unpurified monosaccharides or disaccharides which have already been modified, particularly which have already been oxidised or hydrogenated.
The oxidised monosaccharides and disaccharides undergoing the characteristic step of enzymatic oxidation according to the invention may, in particular, correspond to the products of the oxidation, in one or more places, of the monosaccharides and disaccharides listed above and, in particular, may consist of any of gluconic, glucaric, 5-keto-gluconic, galactonic, galactaric, gulonic, maltobionic or lactobionic acids, said acids being in the free and/or lactonised and/or salt form.
The lactonised form may, by way of example, consist of a gluconolactone, a galactolactone or a gulonolactone.
The hydrogenated monosaccharides and disaccharides may, in particular, correspond to the products of relatively thorough catalytic hydrogenation of the above-mentioned monosaccharides and disaccharides.
It is widely known that monosaccharides or disaccharides, optionally oxidised or hydrogenated already, may be oxidised enzymatically, particularly by oxidoreductases capable of using oxygen as a hydrogen acceptor and hence capable of generating hydrogen peroxide (H2O2) in the reaction medium.
These are, in particular, enzymes of Group 1.1.3 as defined in the document xe2x80x9cENZYME NOMENCLATURExe2x80x9d, revised periodically by the International Union of Biochemistry and Molecular Biology. The nomenclature of such enzymes is recapitulated on pages 55 to 60 of the 1992 edition of said document.
These are, inter alia, enzymes having at least one of the glucose oxidase, hexose oxidase, galactose oxidase or pyranose oxidase activities.
Thus, reference has been made regularly, particularly for about twenty years, to the use of pyranose oxidase (also known, inter alia, as xe2x80x9cglucose 2-oxidasexe2x80x9d, xe2x80x9cpyranose: oxygen 2-oxidoreductasexe2x80x9d or more simply xe2x80x9cP2Oxe2x80x9d) for the enzymatic conversion of monosaccharides, optionally already oxidised, to their equivalents oxidised in the 2 position, and in particular for the conversion of glucose to glucosone or galactose to galactosone. These products are synthesis intermediates of great interest for obtaining products such as fructose, sorbitol, mannitol or tagatose which, on their own or in mixture, have wide fields of application, particularly in the food, pharmaceutical and chemical industries.
For several decades, there has also been wide recourse to the use of glucose oxidase (also known, inter alia, as xe2x80x9cglucose oxyhydrasexe2x80x9d, xe2x80x9cbeta-D-glucose: oxygen 1-oxidoreductasexe2x80x9d or more simply xe2x80x9cGODxe2x80x9d) for the preparation of gluconic acid, in the free, lactonised and/or salt form, from glucose.
However, one of the major disadvantages of the above-mentioned enzymes is that hydrogen peroxide is generated concomitantly and in equimolar quantities with the desired oxidised product. However, it is acknowledged that the presence of hydrogen peroxide is, on the whole, disadvantageous for the activity of an oxidoreductase such as pyranose or glucose oxidase, and efforts are generally made to remove hydrogen peroxide wholly or partly from the reaction medium during the course of the enzymatic oxidation reaction.
Various means of an enzymatic, chemical or physical nature have been proposed for removing or decreasing hydrogen peroxide or at least the adverse effects associated with the generation and presence of this compound in the complex medium which the enzymatic oxidation medium constitutes.
The means most often described consists in the use of catalase in the free or immobilised form, with a view to decomposing hydrogen peroxide enzymatically. This use is described in numerous patents such as the patents WO 81/03664 and WO 81/03666 published in 1981 in the name of STANDARD BRANDS, the subsequent patents U.S. Pat. Nos. 4,351,902, 4,423,149, 4,568,645, 4,569,910, 4,569,913, 4,569,915 and 4,650,758 in the name of CETUS CORPORATION, WO 97/24454 in the name of GENENCOR INTERNATIONAL INC and U.S. Pat. No. 5,897,995 in the name of GIST-BROCADES B.V.
The use of catalase in combination with a glucose oxidase has also been mentioned, where necessary also illustrated by examples, in the articles or patents below:
xe2x80x9cSTABILITY STUDIES ON THE IMMOBILIZED GLUCOSE OXIDASE/CATALASE ENZYME SYSTEMxe2x80x9d, R. S. CARTER et al, ENZYME ENG., (1980), 5, 321-324,
U.S. Pat. No. 4,460,686, published in 1984 in the name of BOEHRINGER INGELHEIM,
xe2x80x9cProduction of gluconic acid with immobilized glucose oxidase in airlift reactorsxe2x80x9d, K. NAKAO et al, CHEMICAL ENGINEERING SCIENCE, (NOV. 1997), VOL. 52, no. 21-22, 4127-4133.
The use of catalase in combination with another oxidoreductase, namely a glycolate oxidase (EC 1.1.3.15), has also been illustrated by examples in U.S. Pat. No. 5,262,314 published in 1993 in the name of E.I. Du Pont de Nemours and Co.
The use of catalase has also been described, inter alia, in the following recent scientific articles:
xe2x80x9cLaboratory procedures for producing 2-keto-D-glucose, 2-keto-D-xylose and 5-keto-D-fructose from D-glucose, D-xylose and L-sorbose with immobilized pyranose oxidase of Peniophoro giganteaxe2x80x9d, A. HUWIG et al, Journal of Biotechnology 32, 309-315 (1994),
xe2x80x9cA Convenient Enzymatic Procedure for the Production of Aldose-Free-D-Tagatosexe2x80x9d, D. HALTRICH et al, ANNALS NEW YORK ACADEMY OF SCIENCES, 864, 295-299 (1998),
xe2x80x9cThe CETUS Process revisited: a novel enzymatic alternative for the production of aldose-free D-fructosexe2x80x9d, C. LEITNER et al, Biocatalysis and Biotransformation, Vol., 00, 1-18 (1998).
Other routes already advocated for the removal/decrease of hydrogen peroxide or of its adverse effects in a reaction medium as envisaged here consist in the use of chemical means such as:
Manganese oxides said to be capable of decomposing H2O2 according to the article xe2x80x9cThe Immobilization of Glucose Oxidase to Manganese Oxidexe2x80x9d, Z. DUVNJAK et al, BIOTECHNOLOGY AND BIOENGINEERING, VOL. XVIII, 737-739 (1976), but which are said not to be sufficiently effective according to U.S. Pat. No. 4,460,686 mentioned above and would not, in fact, be able to act as a substitute for catalase, as follows from the examples of U.S. Pat. No. 5,262,314,
Quinine sulfate or urea which are said to be capable of stabilising glucose oxidase in the presence of H2O2 according to the article xe2x80x9cThe Influence of Peroxidexe2x80x94Stabilizing Agents on Enzyme Deactivation by H2O2xe2x80x9d, Y. K. CHO et al, BIOTECHNOLOGY AND BIOENGINEERING, VOL. XIX, 157-158 (1977),
Activated carbon which is said to be capable of inactivating H2O2 according to the article xe2x80x9cEnzyme Immobilization on Activated Carbon: Alleviation of Enzyme Deactivation by Hydrogen Peroxidexe2x80x9d, Y. K. CHO et al, BIOTECHNOLOGY AND BIOENGINEERING, VOL. XIX, 769-775 (1977) and according to the above-mentioned article by R. S. CARTER,
Alkenes which are said to be capable of xe2x80x9cconsumingxe2x80x9d H2O2 and thus forming recoverable glycols or alkoylene oxides according to U.S. Pat. No. 4,321,324 in the name of CETUS CORPORATION,
Platinum which, in the presence of P2O-producing mycelium and a dilute solution of glucose (2.5%), is said to be capable of decomposing H2O2 according to the patent WO 81/03666 mentioned above, and the use of which, as a substitute for catalase for stabilising glycolate oxidase, is advocated in U.S. Pat. No. 5,262,314 mentioned above,
Stabilisation of pyranose oxidase by amidination according to the above-mentioned U.S. Pat. No. 4,650,758,
Purification of pyranose oxidase by removing the parasitic xe2x80x9cpyranosone dehydratasexe2x80x9d activity according to the above-mentioned U.S. Pat. No. 4,569,913.
Other documents advocate the removal (of the adverse effects) of hydrogen peroxide by combining a physical means (semi-permeable membrane) with an enzymatic means (catalase) or chemical means (alkene) as described in the above-mentioned patents WO 81/03664 and U.S. Pat. No. 4,321,324.
Metals other than platinum have been advocated with a view to stabilising the activity of glucose oxidase (xe2x80x9cGODxe2x80x9d) by decomposition of hydrogen peroxide (H2O2).
This is the case, inter alia, with ruthenium, the use of which is envisaged in the above-mentioned article by R. S. CARTER. This document does not, however, specify the exact conditions of use of this metal in the oxidation reaction medium, and particularly the ruthenium content of said medium, and the ratio of glucose oxidase/catalase activity when the latter is present. On the other hand, this document underlines the gradual xe2x80x9cpoisoningxe2x80x9d of the ruthenium by the gluconic acid generated and draws conclusions about the insufficient protective effect of this metal towards GOD from the point of view of long-term industrial trials.
This insufficient ability of ruthenium to improve the stability of GOD, including immobilised GOD in the presence of catalase, is underlined in column 2, lines 46-51 of the U.S. Pat. No. 4,460,686 mentioned above.
This lack of effectiveness of ruthenium as a substitute for catalase for stabilising an oxidoreductase such as glycolate oxidase follows clearly, moreover, from the U.S. Pat. No. 5,262,314 published subsequently in 1993.
Examples 3, 5 and 13 of said patent show that this metal, even if used in a very large quantity, i.e. greater than 2.5% expressed as dry weight with respect to the dry substance of the reaction medium, or greater than 5% expressed as dry weight with respect to the dry weight of the substrate to be oxidised, does not make it possible to obtain:
high yields of oxidised substrate, these yields always being less than 35%,
a high residual xe2x80x9coxidoreductasexe2x80x9d activity, this activity also always being less than 35%.
This is the case, in particular, when ruthenium is used on an activated carbon support according to example 5 of said patent.
The use of 0.615 g of a catalyst based on 5% ruthenium on activated carbon, that is, about 5.4% ruthenium (dry/dry)with respect to the glycolic acid used, or about 2.7% ruthenium (dry/dry) expressed with respect to the dry substance of the reaction medium, allows at best the following to be obtained after 22 hours"" reaction:
a glyoxylate yield of 32.5%, and
a residual xe2x80x9cglycolate oxidasexe2x80x9d activity of 17%.
Consequently, U.S. Pat. No. 5,262,314 advocates the use of extremely large quantities of xe2x80x9cnon-enzymatic catalystsxe2x80x9d of a very varied nature such as manganese or copper oxides, ruthenium, platinum, palladium, lead, soluble salts or chelates of manganese, copper, nickel, cobalt, zinc, iron or chromium, activated carbon or certain imidazole derivatives, from the point of view of replacing the use of catalase.
More recently, consideration has been given, within the context of reactions carried out in particular pieces of equipment (airlift reactors), to substituting catalase by fine particles of palladium contained in a quantity of 4 wt. % in alginate beads having a density of around 1 g/cm3 and in which GOD is also immobilised.
The exact quantities of palladium used effectively in the reaction medium, particularly in terms of the total dry substance of said medium (including alginate beads) and/or the dry weight of the substrate alone to be oxidised (glucose) are not specified. However, it is pointed out in the last paragraph of the chapter xe2x80x9cOptimal operating conditionsxe2x80x9d on page 4132 of said document that:
the optimum initial concentration of the glucose solution is 10 g/l, and
the optimum volume ratio of the alginate beads with respect to the total volume of beads+glucose solution or xe2x80x9cgel bead contentxe2x80x9d is 0.4.
Consequently, also in view of the presumed density of said glucose solution (≈1 g/cm3) and the very low usual dry substance content of alginate beads intended to contain an enzyme (dry substance less than 20%, generally less than 10%), it is deduced by calculation that the authors advocate a quantity of palladium whatever the circumstances;
greater than 250%, expressed as dry/dry with respect to the quantity of substrate (glucose), and
greater than 15%, expressed as dry weight with respect to the total dry substance of the reaction medium (including beads).
The overall result of the above is that, in the case of glucose oxidase and glycolate oxidase, metals such as, inter alia, ruthenium or palladium cannot be effective substitutes for catalase unless, optionally, they are used in necessarily very high and hence extremely expensive quantities.
Moreover, in the case of pyranose oxidase, there is no option but to ascertain that the most recent prior art, represented by the examples of the above-mentioned CETUS CORPORATION patents and scientific articles, envisages in practice only catalase as the principal means of removing hydrogen peroxide and its adverse effects.
In particular, catalase may be physically combined with the oxidoreductase (pyranose oxidase, glucose oxidase in particular) by way of enzymatic complexes naturally present in various microorganisms or pre-mixtures of enzymes of different origins, but also by means of co-immobilisation on the same support, these variants being described, for example, in the U.S. Pat. Nos. 4,569,910, 4,650,758, 4,351,902 and 5,897,995 mentioned above.
Catalase may also be combined with bovine serum albumin (xe2x80x9cBSAxe2x80x9d acting as a stabiliser or protector of the oxidoreductase as described in the articles by HALTRICH and LEITNER mentioned above. The precise mode of action of BSA is poorly understood but, according to LEITNER this protein could, in particular, protect pyranose oxidase and also catalase from the adverse effects of glucosone. Other protective agents may consist of casein, methionine, quinine or mannitol, sorbitol or glycerol according to the U.S. Pat. No. 5,897,995.
Whatever the case, it is known that catalase is also inactivated by hydrogen peroxide and that it is necessary, in practice, to use it in large quantities, in any case in a large excess with respect to the quantities of oxidoreductases used in conjunction therewith.
The patents WO 97/24454 and U.S. Pat. No. 5,897,995 published very recently envisage ratios between the number of units of catalase and the number of units of oxidoreductase (glucose oxidase) used in the reaction medium (hereinafter known as the C/O ratios) of the order of 40-80 (WO 97/24454) or even equal to 140 (U.S. Pat. No. 5,897,995).
Finally, the articles by HALTRICH and LEITNER mentioned above advocate a CIO ratio of 1000, combined with the use of BSA in a quantity of 5 mg/ml of reaction medium, with the possibility of recycling the oxidoreductase (pyranose oxidase) and BSAxe2x80x94unlike the catalase used which is unstablexe2x80x94after their extraction from the reaction medium by ultrafiltration.
Another teaching by LEITNER relates to the fact that the platinum deposited on activated carbon cannot, in fact, be used as a substitute for catalase because its use results in a loss of more than 95% of the P2O activity under standard operating conditions.
Consequently, in spite of the cost of using catalase, no serious consideration was given, until then, to questioning the growing interest and use.
After considerable research, a new means has now been developed for the effective preparation of organic materials oxidised enzymatically such as, for example, glucosone or gluconic acid, said means making it possible, if desired, to obtain said materials in the presence of reduced or even zero quantities of catalase, and/or protective agent such as BSA.
The Applicant company has, in particular, found that, surprisingly and unexpectedly, ruthenium and palladium in small quantities could, unlike platinum in particular, be used advantageously as a substitute for catalase and allow an oxidoreductase such as pyranose or glucose oxidase to act rapidly with a high yield and excellent selectivity.
More specifically, the present invention relates to a process for the conversion of an organic material comprising an oxidation step during which an organic material undergoes the oxidising action of an enzymatic means capable of generating hydrogen peroxide, this process being characterised in that said oxidation step is carried out wholly or partly in the presence of 0.001% to 1% of a metal selected from ruthenium, palladium and mixtures thereof, these percentages being expressed as total dry weight of ruthenium and palladium with respect to the total dry weight of the reaction medium.
Advantageously, said metal is present in an amount in the range 0.005% to 0.2%
As mentioned, it is not obligatory that said metal be present in the reaction medium perfectly simultaneously with the means of enzymatic oxidation; one may be introduced into the reaction medium and/or withdrawn from the reaction medium before the other. Ruthenium and/or palladium may be introduced in any suitable forms, particularly in the immobilised form on a support, obtained by any known method, particularly impregnation or ion exchange. The support may be composed, for example, of activated carbon, peat, zeolite, titanium dioxide or a synthetic polymer of the carbon fibre type. An undeniable advantage of the process claimed here is that it allows the recycling of said metal, which is generally impossible with an ordinary catalase.
The organic materials, particularly of a saccharide nature, that may be used as a substrate for at least one of the means of enzymatic oxidation envisaged here have been described above and relate in particular to monosaccharides, disaccharides and their respective derivatives, for example, those which have already been oxidised.
The term xe2x80x9cmeans of enzymatic oxidationxe2x80x9d within the context of the present invention means, in particular, the oxidoreductases of group 1.1.3 within the meaning of the document xe2x80x9cENZYME NOMENCLATURExe2x80x9d mentioned above and mixtures of at least any two of these enzymes, it being specified that said enzymes may be used in the free form or immobilised on a support, such as they are, in the partly or wholly purified form and/or by means of organisms which synthesise said enzymes.
By way of example, the means of enzymatic oxidation may be chosen from enzymes or mixtures of enzymes having at least one of the activities of glucose oxidase, hexose oxidase, galactose oxidase, pyranose oxidase, L-sorbose oxidase, cellobiose oxidase, L-gulonolactone oxidase, L-galactononolactone oxidase, alcohol oxidase, secondary alcohol oxidase or (S)-2-hydroxyacid oxidase, and their respective equivalents, current or future.
Advantageously, the means of enzymatic oxidation has at least one of the glucose oxidase, hexose oxidase, galactose oxidase or pyranose oxidase activities.
It may be, in particular, a preparation based on partly or wholly purified enzyme(s) in the free form or immobilised on a support, having a pyranose oxidase and/or glucose oxidase activity, or based on organism(s), in the free form or immobilised on a support, which synthesise this type of enzyme(s).
Such organisms, enzymatic preparations and enzymes which may advantageously be recycled, are described in the above-mentioned patents and scientific articles which form an integral part of the present description. They may, by way of example, be derived from (consist of) any of the following organisms:
Polyporus obtusus, Trametes multicolor, Coriolus versicolor, Lenzites betulinus, Oudemansiella mucida, Aspergillus flavus capable of synthesising, inter alia, pyranose oxidase,
Aspergillus niger, Aspergillus oryzae capable of synthesising, inter alia, glucose oxidase, and all their taxonomic and/or functional equivalents, current or future, of natural origin or otherwise, resulting in particular from treatments, mutations or genetic manipulations.
Preferably, the means of enzymatic oxidation has a pyranose oxidase activity.
According to a variant of the process according to the invention, the enzymatic oxidation step is carried out in the presence of a ratio of catalase/oxidoreductase activity(C/O ratio) as specified above, of less than 1000, and particularly
less than 500, preferably less than 200, if the oxidoreductase consists of pyranose oxidase, or
less than 40 if the oxidoreductase consists of glucose oxidase.
It should be recalled here that a preparation of oxidoreductase may, despite continuous efforts to purify the enzyme, exhibit simultaneously a relatively high catalase activity which is relatively intimately associated with it physically. This is due in particular to as yet incomplete purification and/or recycling from a medium which also contained catalase. The use of said oxidoreductase may therefore involve, particularly due to its recycling, the concomitant use of an xe2x80x9cendogenousxe2x80x9d catalase activity which should be distinguished from an xe2x80x9cexogenousxe2x80x9d catalase activity, i.e. supplied in a predetermined manner, independently of the oxidoreductase activity, simultaneously or otherwise with the latter.
According to a variant of the process according to the invention, the enzymatic oxidation step is carried out in the absence of any exogenous supply of catalase activity.
According to another variant, the means of enzymatic oxidation and the metal (ruthenium and/or palladium) are immobilised on the same support.
The concentration of the organic material used as a substrate of the means of enzymatic oxidation is not subject to any particular constraint as regards the prior art. The organic material may be supplied in particular in the form of a solution having a dry substance (xe2x80x9cDSxe2x80x9d) content in the range 2% to 70%, particularly from 4% to 60%. It may be, for example, a glucose solution having a DS of more than 5%, particularly in the range 6% to 55%, which may be used as a substrate of pyranose oxidase or glucose oxidase. In practice, the enzymatic oxidation reaction is generally carried out at a temperature in the range 15xc2x0 C. to 60xc2x0 C., for example, in the range 20xc2x0 C. to 35xc2x0 C. if the means of enzymatic oxidation consists of pyranose or glucose oxidase.
The other reaction parameters are those generally found in the literature, including those concerning aeration, agitation and the pH of the reaction medium.
The Applicant company has found, however, that in the case of pyranose oxidase, the pH of the reaction mixture was advantageously above about 5.5 and particularly in the range about 5.6 to 6.5.
As mentioned, the process according to the invention makes it possible, remarkably, to obtain an oxidised material such as glucosone with a very high yield and selectivity from glucose within very short periods of time. If the means of oxidation has pyranose oxidase activity, the process according to the invention may therefore be characterised advantageously by the fact that the enzymatic oxidation step has a duration of less than 6 hours, preferably less than 5 hours.
Consequently, a new, simple, effective, inexpensive and very selective means is now available for obtaining oxidised organic materials.
This means is particularly suitable for the preparation of a saccharide composition containing at least one monosaccharide or a disaccharide, preferably selected from the group comprising glucose, galactose, mannose, xylose, sorbose, maltose, lactose and any mixtures of at least any two of these products, oxidised in at least one place and optionally lactonised, particularly for the preparation of a composition based on at least one product selected from the group comprising glucosone, galactosone, xylosone, glucosyl-glucosone, 2,5-diketo-fructose, gluconic acid, galactonic acid, 2-keto-gluconic acid, 2-keto-glucaric acid, 2,5-diketo-gluconic acid, isoascorbic acid, said acids being in the free and/or lactonised and/or salt form, and any mixtures of at least any two of said products.
Due, in particular, to their purity, the oxidised organic materials obtained in accordance with the invention may undergo advantageously, if desired, one or more subsequent modification steps, particularly of a chemical nature.
The present invention relates in particular to a process as described above, characterised in that it comprises, subsequent to the enzymatic oxidation step in the presence of ruthenium and/or palladium, at least one additional step involving the reduction or oxidation of the enzymatically oxidised, optionally purified organic material.
The reduction step may take place enzymatically as described in the articles by HALTRICH and LEITNER mentioned above. Advantageously, it is a catalytic hydrogenation step as described in the patents WO 81/03666, U.S. Pat. No. 4,321,324 and U.S. Pat. No. 4,423,149 mentioned above.
By way of example, the glucosone preparation obtained from glucose in the presence of pyranose oxidase and ruthenium and/or palladium may, particularly due to its low content of formic acid or of other species acting as catalyst poisons, undergo advantageously an additional oxidation or catalytic hydrogenation step.
Another undeniable economic advantage of the process according to the invention is, moreover, that it allows the recycling of the ruthenium and/or palladium used during the enzymatic oxidation step for the purpose of using it during a subsequent catalytic hydrogenation step.
According to one variant, this process is characterised, therefore, in that it comprises an additional catalytic hydrogenation step, continuous or batchwise, this being preferably also carried out in the presence of ruthenium and/or palladium recycled from the previous enzymatic oxidation step or not recycled.
This process may therefore be used advantageously for the preparation of a saccharide composition containing at least one product selected from the group comprising fructose, sorbitol, mannitol, tagatose, hydrogenated or not hydrogenated, glucosyl-sorbitol, glucosyl-mannitol and any mixtures of at least any two of these products.
According to another variant of the process according to the invention, the enzymatic oxidation step is carried out continuously. Thus, the organic material that has to undergo said step is introduced continuously into the reaction medium and the oxidised organic material that has undergone said step, for example, glucosone, is drawn off, also continuously, from said reaction medium.
In this case, the ruthenium and/or palladium (on a support) may be used advantageously in the form of granules which are placed in a perforated and generally fixed vessel immersed in the reaction medium. The metal granules may thus remain in continuous contact with the reaction medium.
Moreover, the oxidised organic material may, according to another variant, advantageously undergo, likewise continuously or batchwise, at least one subsequent filtration step, for example, microfiltration and/or ultrafiltration, with a view in particular to allowing the continuous or batchwise recycling of all or part of the means of enzymatic oxidation and/or the metal (ruthenium and/or palladium) used beforehand. The oxidised organic material thus purified, for example, glucosone, may then undergo, as already mentioned, a subsequent, particularly continuous, hydrogenation step.
The present invention will be described in more detail with the aid of the examples below which are in no way restrictive.